The term "arc welding" applies to a large and diversified collection of welding processes that use an electric arc as the source of heat to melt and join metals. The arc is struck between the workpiece and an electrode that is manually or mechanically moved along the joint, or that remains stationary while the workpiece is moved underneath it. The electrode will either be a consumable wire or rod, or a nonconsumable rod made of, for example, carbon or tungsten which carries the current and sustains the electric arc between its tip and the workpiece. When a nonconsumable electrode is used, a separate rod or wire can supply filler metal if needed.
Of particular importance for purposes of the instant invention, the gas-tungsten arc welding process ("GTAW," hereinafter, or alternatively, "TIG" welding as it is sometimes called) uses a nonconsumable tungsten electrode for one pole of the arc. The tungsten electrode, and the weld zone (i.e., the area being welded) are shielded from the atmosphere by an inert gas, such as argon or helium, which gas acts to reduce oxidation of the electrode. The heat necessary for fusion (mixing or combining of the molten metals) is provided by an arcing electric current between the tungsten electrode and the base metal. The arc fuses the metal being welded as well as filler metal if it is used. The gas shield protects the electrode and weld pool and provides the required arc characteristics. This type of welding is usually done with a single electrode. However, it may alternatively be done with several electrodes.
The GTAW process may employ either direct current (DC) or alternating current (AC), DC straight polarity current being generally preferred for most applications. Regardless of current type, a constant current welding supply is required. In addition, a high frequency oscillator is usually an integral part of the power supplies intended for GTAW. High frequency can be employed with DC to initiate the arc instead of touch starting so as to minimize tungsten electrode contamination. It is therefore normally turned off automatically after arc ignition. High frequency is employed with AC for initiating the arc and to insure its re-ignition at each half cycle while welding.
The basic equipment needed for GTAW welding consists of a welding torch, welding power supply, and a source of inert gas with suitable pressure regulators and flowmeters. Electric power requirements depend upon the thickness to be welded and range from 8 kW for a 200 amp DC unit to 30 kW for a 500 amp AC balanced wave unit. Portable engine driven power supplies are available, as well as are standard line voltage units.
Cold wire feeders are used in manual, semiautomatic and fully automatic operations where filler wire application is required. They are adaptable to both hard (steels, stainless steel, etc.) and soft (aluminum, magnesium, etc.) wires. Various types of drive mechanisms and guides can be used to adapt the units to different diameters of filler wire.
Box header welding, or tube-to-tube sheet welding as it is also called, presents certain logistical problems which are not present in general welding applications, tube-to-tube welding being commonly necessary during the construction of heat exchangers, boilers, and condensers. In this application, several tubular members are affixed to a flat sheet with holes therein which forms one face of a "box". The tubular members are aligned so that they extend perpendicularly from the face of the box and mate with one of the tube holes in that face. The opposite face of the box contains an equal number of threaded and tapped access holes or "plug holes" passing therethrough. As the name suggests, after the tubes are affixed to the box header, plugs are installed in these holes and the unit is then pressurized to test the integrity of joint between the tubular members and the flat sheet. The tubes and the material to which they are welded might be made of almost any sort of combinations of metals.
Of particular importance for purposes of the instant invention is the difficulty in performing the weld in question when it must be done inside of the boxheader. The preferred way of doing this is to insert a welding apparatus through an opposing plug hole and over to the opposite side where the weld is performed. For example, Kiefer et al., U.S. Pat. No. 4,357,515, teaches a complex welding torch made of a high temperature plastic which contains, among other things, a fiber optic bundle that allows video display of the welding process. Jusionis et al., U.S. Pat. No. 5,221,818, discloses a box header welding apparatus that contains a rotating mandrel and a centering mechanism that helps to assure proper radial alignment of the welding torch with respect to the tube. Both of these inventions, however, are complex pieces of equipment that are not capable of moving from hole-to-hole very quickly, which results in increased time and cost to perform a job. Additionally, inventions of this type tend to limit the movability and accessibility of the torch within the box header such that a skilled welder, given the proper tool, can in most cases do the job better and faster.
Other popular general approaches to box header welding that have been used in the past include stick welding (i.e., shielded metal arc welding) and fusion methods (e.g., roll expanding or automatic fusion methods). However, these approaches have proven to be less than satisfactory in practice, as the joints have tended to develop leaks over the long term, and this is especially true in the case of fusion methods. Additionally, customers are demanding with increasing frequency the use of strength welds, which generally eliminates fusion joining methods from consideration.
Thus, what is needed is an apparatus for tube to tubesheet welding that overcomes the disadvantages of the prior art and provides skilled craftsmen with an effective way to perform tube-to-tubesheet welding within the confines of a box header. Accordingly, it should now be recognized, as was recognized by the present inventor, that there exists, and has existed for some time, a very real need for a tube-to-tubesheet welding system that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.